Sewage is composed of the liquid and water-carried wastes from residences, commercial buildings, industrial plants, and institutions, together with any groundwater, surface water and storm water which may be present. The terms “wastewater” and “sewage” are sometimes used interchangeably. The composition of sewage depends upon its origin and the volume of water in which the wastes are carried. Sewage which originates entirely from residential communities is made up of excreta, bathing and washing water, and kitchen wastes. Other wastes can be present from rural/agricultural sources and/or industrial or commercial establishments.
Modern sewage treatment is generally divided into three phases: primary, secondary and tertiary. Each of these steps produces sludge, which can be disposed of or used for various purposes. Sludge is the semiliquid mass removed from the liquid flow of sewage. Sludge will vary in amount and character with the characteristics of sewage and plant operation. Sludge from primary treatment is composed of solids usually having a ninety-five percent moisture content. The accumulated solid materials, or sludge, from sewage treatment processes amount to fifty to seventy pounds per person per year in the dry state or about one ton per year in the wet state. Sludge is highly capable of becoming putrid and can, itself, be a major pollutant if it is not biologically stabilized and disposed of in a suitable manner. Biological stabilization can be accomplished by either aerobic or anaerobic digestion. After digestion, sludge-drying beds are usually used.
In modern sewage treatment plants, mechanical dewatering of sludge by vacuum filters, centrifuges, belt presses, or other devices is becoming widespread. Many kinds of sludges are difficult to dewater with conventional dewatering equipment such as chamber filter presses, belt filter presses and other similar equipment. Therefore, prior conditioning is necessary so as to improve the capability for dewatering. In the past, such conditioning is generally achieved by the addition of one or several chemicals acting as flocculation agents. The capability for dewatering which has been achieved by conditioning will depend strongly upon the quantity, size and especially the structure and stability of the formed floc particles. Unfortunately, the use of such flocculation agents is a rather expensive process and, as such, it is considered desirable to use flocculants very sparingly.
One of the major problems associated with prior attempts to dewater sludge prior to introduction into such belt presses is the fact that a great deal of water is retained within the interstitial structures of the organisms. Typically, belt presses will only extract external water from the cell membranes. The belt presses are generally ineffective in extracting the interstitial water accumulated within the cell membranes. As a result, belt presses have been generally ineffective at removing a large amount of the water from the sludge. In order to fully remove the water from the sludge, it would be necessary to gain access to the interstitial water accumulated within the cell membrane walls of the cells within the wastewater sludge.
In the past, various patents have issued relating to dewatering processes. For example, U.S. Pat. No. 6,101,738, issued on Oct. 15, 2000 to G. Gleason, describes a sludge dewatering system in which the sludge is dewatered by introducing pressurized air into the sludge. The air serves to strip the sludge of its water and thereby increase the total solids captured with respect to time. The pressurized air is applied through the surfaces of the belt press so as to “blow” through the sludge accumulated upon the belt press.
U.S. Pat. No. 6,051,137, issued on Apr. 18, 2000 to F. D. Deskins, describes a process of dewatering primary-treated sewage which includes the step of mixing the sewage with a coagulant or flocculant, such as an activated polymer. The sewage is then mixed and flocculated at conditions which involve extensive mixing turbulence of the sewage whereby part of the sewage is recycled so as to be subjected to mixing and flocculating. The pH of the sewage is chemically adjusted to the basic pH range. The sewage is applied to a sand bed whereby the flocculated solids in the sewage are separated from the liquid in the sewage. The flocculated solids located on the top of the sand bed are then air dried.
U.S. Pat. No. 5,961,827, issued on Oct. 5, 1999 to A. Bahr, describes an apparatus for dewatering sludge which includes a sludge chamber provided with filter areas having at least one sludge inlet connected to a device for forming a hydrostatic filtration pressure. The sludge chamber is formed by pressure plates that can be pressed against each other to create mechanical dewatering pressure. There is a predewatering stage containing filter areas which forms a compensation container and which is connected to a continuous sludge feed.
U.S. Pat. No. 5,885,445, issued on Mar. 23, 1999 to Andrews et al., describes a belt press for dewatering sludge. The press includes a camera to monitor the physical operation of a gravity belt section of the press. A numerical control device utilizes electromagnetic radiation received from the gravity belt section to control the physical operation of the belt section.
U.S. Pat. No. 5,770,056, issued on Jun. 23, 1998 to F. D. Deskins, is related to the later issued U.S. Pat. No. 6,051,137 and also describes the process of dewatering primary-treated sewage by adding a coagulant or a flocculent to the mixed sewage.
U.S. Pat. No. 5,366,622, issued on Nov. 22, 1994 to S. Geyer, describes a process for the dewatering of sludge which involves the addition of a flocculant to the sludge suspension. A pressure pipe is placed between a feed pump and dewatering equipment. There are a number of dosing points located along the pressure pipe so as to allow for the introduction of the flocculant at desired location during the feed of the sewage toward the dewatering equipment.
U.S. Pat. No. 4,767,537, issued on Aug. 30, 1988 to H. F. Davis, teaches the dewatering of sludge by the addition of nitrate ions to the treated sludge so as to generate microscopic bubbles of nitrogen gas that adhere to the sludge floc particles. This causes a reduction in density of the particles which, in turn, causes the particles to float to the top of a thickening tank. The process separates the sludge into an upper thickened layer and a lower free water layer.
The present inventor is also the inventor on several prior issued U.S. patents for processes for treating waste sludge of biological solids. In particular, U.S. Pat. No. 5,635,069, issued on Jun. 3, 1997, describes a process for treating waste sludge of biological solids that includes the steps of mixing the sludge with an oxide-containing chemical and sulfamic acid so as to elevate a temperature of the sludge, pressurizing the mixed sludge to a pressure of greater than 14.7 p.s.i.g., and discharging the pressurized mixed sludge. This sludge has a water content of between 5 and 85 percent. The oxide-containing chemical and the acid are reacted with the sludge so as to elevate the temperature of the sludge to between 50° C. and 450° C. The pressurized mixed sludge is flashed across a restricting orifice or passed into a chamber having a lower pressure. The evaporated liquid component can be condensed and used as part of the process.
U.S. Pat. No. 5,868,942, issued on Feb. 9, 1999, teaches a process for treating a pathogen-containing sludge of biological solids. This process includes the steps of mixing the sludge with calcium oxide, ammonia and carbon dioxide so as to elevate a temperature of the mixed sludge to between 50° C. and 140° C. and to elevate a pH of the mixed sludge to greater than 9.8. The process also includes the steps of pressurizing the mixed sludge to a pressure of greater than 14.7 p.s.i.a., and discharging the pressurized mixed sludge. The sludge has a water content of between 65% and 94% by weight. The pressurized mixed sludge is discharged by flashing the sludge across a restricting orifice and by evaporating a liquid component of the flashed sludge.
U.S. Pat. No. 6,056,880, issued on May 2, 2000, describes a process for treating a waste sludge of biological solids which includes the steps of blending the sludge with an acid, mixing an oxide-chemical with the blended sludge so as to cause a reaction which elevates a temperature of the sludge, pressurizing the mixed sludge to a pressure of greater than 14.7 p.s.i.a. and retaining the mixed sludge at such pressure for a period of time of no less than 15 seconds, and discharging the pressurized mixed sludge. The step of pressurizing is accomplished by passing the mixed sludge as a flow into the pipe. An immersion heater or a magnetic field is installed so as to apply heat directly to the mixed sludge as the mixed sludge passes through the pipe.
U.S. Pat. No. 6,214,064, issued on Apr. 10, 2001, describes a process for making a fuel product from coal fines and sewage sludge. In this process, the sewage sludge is blended with an acid and mixed with an oxide-containing chemical so as to cause a reaction which elevates a temperature of the sludge. This mixtures is then pressurized to a pressure of greater than 14.7 p.s.i.a. for a period of time of no less than 15 seconds. The pressurized mixed sludge is mixed with coal fines. This mixture is then solidified. The acid that is introduced is sulfamic acid.
It is an object of the present invention to provide a process for enhancing the digestion and for the dewatering of sludge to allow for the removal of the interstitial water from the cell membranes within the sludge and to decrease the amount of sludge removal.
It is another object of the present invention to provide a process which will reduce the water content of the sludge prior to passing to the belt press or other dewatering equipment.
It is another object of the present invention to provide a process which provides an acid treatment during the process to enhance carbon dioxide release.
It is still a further object of the present invention to provide a process for the dewatering of sludge which is extremely cost effective.
It is still a further object of the present invention to provide a process for the dewatering of sludge which achieves higher percent solids.
It is still another object of the present invention to provide a process which decreases the organic particle size of sludge in order to increase the conversion rate of carbon dioxide or methane.
It is a further object of the present invention to provide a process for the dewatering of sludge which is easy to use and install.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.